Check out my car and talk to me about 318/340 swap

To the OP. If I were in your position I'd drive the car with the 318 while accumulating the parts to build the 340. I'd put a set of light weight flat top pistons in it, Ross makes some nice ones. A small cam, and a RPM Airgap manifold with a 750 DP.

Years ago I rebuilt the 340 I had in my 72 Duster, used a set of the heavy OEM 68-70 pistons, IIRC a MP 280/474 hydraulic, a 650DP the original 340 intake and manifolds. The car had 3.55 gears and ran very well. It was a hoot to drive.